Low temperature X-ray investigation of NH₄Br

The lattice parameters of NH₄Br have been measured as a function of temperature from 5 to 150°K, a range which includes the region of the transition between cubic and tetragonal ordered phases. The low-temperature CsCl-type phase exhibits normal thermal expansion up to ~ 108°K on warming, while the tetragonal diffraction pattern persists down to ~ 80°K on cooling. The present data are combined with previous measurements between 150 and 300°K to yield an overall picture of the variations associated with changes in ordering.